A wireless communication unit usually forms part of a wireless communication system. The wireless communication unit communicates through a wireless communication network, which also forms part of the wireless communication system.
The wireless communication network normally comprises a network of base stations. Each base station enables communication within an area referred to as a cell-site. Each cell-site may comprise multiple sectors. There are usually three sectors in a cell-site. Each sector may be served by a dedicated antenna, co-located with the base station.
In some countries, it is a legal requirement that wireless communication systems be able to provide accurate information about the location of wireless communication units. This information may serve, for example, to expedite the arrival of assistance to a user of a wireless communication unit who calls the emergency services, using the ‘911’ number in the United States.
Information about the location of a wireless communication unit can be derived in many ways. At any particular time, various forms of measurement information may be available from:    (i) The wireless communication unit;    (ii) The wireless communication network, and particularly from one or more base stations with which the wireless communication unit is communicating; or    (iii) Both of (i) and (ii).
This measurement information can be processed to provide an estimate of the location of the wireless mobile communication unit.
Considering the measurement information in more detail, this information may be available either:
(i) Directly. This means that the information is included in the measurement made. The measurement may be made either by the wireless communication unit, or by another part of the wireless communication system, such as the wireless network.
(ii) Indirectly. This means that the information is derived from the measurements made. An example would be an estimate of the distance between a wireless communication unit and the base station of a wireless communication system. Such an estimate might be calculated by multiplying the speed of propagation of the signal by a measured time difference between transmission and receipt of a signal.
So some or all of the following mobile measurement information may be available:
(i) The absolute distance(s) from the wireless communications unit to one or more network sectors.
(ii) Differential distances between the wireless communications unit and one or more pairs of network sectors.
(iii) Received signal powers recorded by the wireless communications unit from one or more network sectors.
(iv) Received signal-to-noise ratio measurements recorded by the wireless communications unit from one or more network sectors.
In addition, the following network configuration data may be available:
(i) Antenna locations per sector. This information may be provided in latitude and longitude, or as ‘Easting’ and ‘Northing’ directions, or the equivalent.
(ii) Antenna properties. This information may include height above ground, azimuth, tilt, horizontal and vertical beam pattern, transmitted power levels for control and traffic channels.
So prior art geo-location methods have as their inputs various forms of directly- and indirectly derived measurement information. That measurement information may have come from the wireless communication unit, the wireless communication system, or both. A combination of the mobile measurement information and network configuration data is often used to provide geometric interpretations of the mobile device location.
Network configuration data is, however, often in error. For example, the data on the locations of cell sites can be off by hundreds of meters or more. Azimuthal angles of antennae can be off by tens of degrees. These inaccuracies can impede the ability to geo-locate wireless communication units in the network. In addition, inaccurate network configuration data makes it difficult to perform network planning and optimization effectively.
FIG. 1 shows a simple example of a wireless communication system 100. Wireless communication unit 110 communicates with base station 120. Base station 120 is located at coordinates (xB, yB). Base station 120 is one of many base stations that together comprise the wireless communication system 100. Base station 120 is the nearest base station to wireless communication unit 110, and is located a distance ‘R’ from wireless communication unit 110.
FIG. 1 shows one problem with prior art geo-location methods. If the base station 120 has an omni-directional antenna, and no other measurement information is available, then any attempt to provide a single estimate of the location of the wireless communication unit 110 is very difficult. The information available from the wireless communication unit 110 and/or from base station 120 allows prior art geo-location methods to calculate only the absolute value of distance R. This distance is measured relative to the known location of base station 120, at (xB, yB). However, all that is known is that the wireless communication unit 110 is located somewhere on a circle of radius R, centred on (xB, yB), which is the circle 130 in FIG. 1.
The single estimate of the location would have to be a point chosen at random on the circle 130. The error in this estimate could be up to 2R, because the wireless communication unit might in fact be located on circle 130 at a point diametrically opposite to the estimated location.
In general terms, prior art geo-location techniques usually deliver a single point in space as their estimate of the location of the wireless device. This point may, for example, be described by an x coordinate and a y coordinate, as in FIG. 1. However, such an approach does not give the user information about the reliability of the estimate of the location. Reliability, in this example, means both ‘accuracy’ and ‘precision’.
‘Accuracy’ concerns whether the estimated location is the correct one, or not. In the example of FIG. 1, it was explained that the measurement of the location of mobile communication unit 110 might be inaccurate by as much as 2R.
‘Precision’ is the exactness of the measurement. FIG. 1 assumes that the distance R could be determined very precisely, i.e. that an exact value for R could be derived. However, there is in fact an error range associated with the measurement of R itself. The measurement of R is, in reality, imprecise. This is explained further in connection with FIG. 2 below.
Both accuracy and precision depend on the type and quality of data on which the measurement estimate is based. If the measurement estimate is derived using network configuration data, then any error in that data will be likely to lead to inaccuracy or imprecision.
FIG. 2 illustrates the imprecision in the measurement of R. FIG. 2 corresponds generally to the arrangement of FIG. 1. A base station 220 located at point (XB, YB) might measure the distance to a communication unit 210 as R. Circle 230 shows the locus of all points at the distance R, which is the distance from base station 220 at which the mobile is most likely to be located. Due to the measurement collection process, however, the user 210 might actually be at a distance of between (R−e1) and (R+e2) from base station 220. So it is not possible even to say with certainty that the user 210 lies on circle 230.
FIG. 3 illustrates a situation where the network configuration data includes incorrect data about the location of the base station, which causes further inaccuracy. The elements of FIG. 3 correspond to the similarly numbered elements of FIG. 2. However, (XB, YB) are incorrect co-ordinates for the location of the base station. Point (XR, YR) shows the correct location for base station 320. Point (XR, YR) is at a distance d from point (XB, YB).
In the arrangement of FIG. 3, the wireless communication network would calculate the position of communication unit 310 as being at a distance R from the base station 320. Similarly to the situation in FIG. 2, errors arise due to the measurement collection process. The distance d adds to these errors. As a consequence, including all sources of error, the user might actually be at a distance of between (R−e1−d) and (R+e2+d) from base station 320.
An error function describes the probability that communication unit 210 or 310 is located at each particular distance from base station 220 or 320. The error function is usually complex. However, the details of the error function can be determined through a variety of means. One option, which is known in the prior art, is to place calls from a small number of known locations, and compare the measurement data with the known locations. A large number of test calls would be needed to provide a significant reduction in the size of the errors described in connection with FIGS. 2 and 3. Such testing would therefore be expensive, and might have to be repeated frequently.
Summing up the discussion of FIGS. 1-3, there is both inaccuracy and imprecision in the measurement of the location of a mobile communications unit in prior art wireless cellular mobile communications systems. These problems may be significant in many situations, for example:
(i) When received signal strength from an omni-directional antenna is the only measurement data on which an estimate of the position of the mobile communications unit can be made; and
(ii) When network configuration data is in error.
In the preceding discussion and the discussion that follows, the term ‘communicating’ includes a variety of forms of communication. These forms include, but are not limited to, speech or data communication sessions on traffic channels, and communication on the control channel. So, for example, the communication may not require the user of a mobile telephone to actually place or receive a call. The communication may involve, for example, only the intermittent receipt by a mobile telephone of data, for example over the wireless communication system's control channel.
Prior art arrangements for the correction of network configuration data typically involve sending technicians to the cell sites in question. The technicians may use Global Positioning System receivers to determine the latitude & longitude of a base station. They may also use inspection materials, such as compasses and maps, in order to determine antenna azimuths and tilts. These quality control inspections are well known by those practiced in the art. In order to check a whole wireless communications network, it is necessary to send technicians to the tens of thousands of cell sites that a wireless service provider owns. The time and effort involved in such a major program of visits leads to a situation where these site visits tend not to be done very often. Although the installation of a cell site does involve the recording of information that becomes part of the network configuration data, this information can be erroneous. Such errors arise due to the data typically being entered manually. A further source of error arises due to changes made to the network over time, which may not be recorded. Such changes result from optimization of the network and other important objectives.
Prior art U.S. Pat. No. 5,293,642 (Lo) describes a variety of approaches to estimating the location of a mobile communication unit. The approaches involve calculating a probability density function for the location of the mobile communication unit. A mobile station may be in communication with two or more base stations. In this case, several probability density functions can be calculated, each describing the location of the mobile on the basis of measurements received from one of the base stations. These probability density functions may be combined, to provide a joint probability density function.
Prior art United States patent application US2008080429 describes a process of minimum variance location estimation in wireless networks. A probability density function is calculated, and a ‘probability surface’ is derived from the probability density function. A mean location of a wireless node is calculated from the probability surface.
The prior art arrangements of these two documents provide an alternative to placing test calls within a network. However, the accuracy of the location information that they provide will still be compromised by any errors in the network configuration data that is available to them.